1. Field of the Invention
This invention relates to a plug valve and to a method of making such valve.
2. Prior Art Statement
It is known in the art to provide a plug valve for controlling fluid flow therethrough with such valve having an outer housing or body which has a flow passage extending through the housing and including an inlet and an outlet interconnected by a bore disposed therebetween. The bore has a central axis disposed substantially perpendicular to the flow passage and a plastic sleeve is disposed within the housing and lines the bore. The plastic sleeve has a pair of aligned ports therein at diametrically opposed locations and the ports are aligned with the inlet and outlet to enable fluid flow through the passage. In such a plug valve a rotatable plug member or plug is disposed within the sleeve and the plug has an opening therethrough which is adapted to be aligned in and out of registry with the inlet and outlet in accordance with rotation relative to the housing. In such a prior valve, the plastic sleeve provides the primary fluid seal for the valve. Further, the holding means generally consists of what will be referred to as body lips defined as an integral part of the valve body and such lips are associated with the inlet and outlet in the housing. The lips of a typical known valve shield the plastic sleeve liner from direct impingement of fluid flowing through the valve and protect it from any abrasive material entrained in the fluid. The body lips help prevent cold flow of the sleeve liner and prevent rotation of the sleeve liner relative to the valve body whenever the plug member is rotated between open and closed positions.
It is common practice in such a prior plug valve to use a sleeve liner of polytetrafluoroethylene or equivalent material and with such a sleeve liner there is a strong tendency for the liner to be drawn or deflected toward the center line of the valve when the rotatable plug member is in a partially or nearly closed position. When the plug member is in this nearly closed position, the velocity of the fluid proximal to the side of the port is markedly increased with a resultant decrease in pressure, whereby this reduction in pressure tends to provide movement of the plastic sleeve into the flow passage by pulling, dislocation, or deflection. The body lips also serve to prevent such movement of the plastic sleeve under these flow conditions.
In the past, these body lips have been made as an integral part of the valve outer housing or body, as by casting. However, the casting of these lips in the valve body increases the manufacturing cost of such valve body and makes machining of the valve body very expensive. Further, in a valve housing or body having body lips, it is very difficult to precisely machine exposed sleeve engaging surfaces of the usual high pressure sealing ribs of the plug valve to assure proper sealing of the valve due to the difficulty in providing a machining action with the body lips present. In addition, the body lips also cause problems in the assembly of the plastic sleeve liner in the valve body and these problems add to the cost of the overall prior art plug valve.
It is also common practice with prior art plug valves to replace the entire outer housing or body once the body lips become damaged or defective. However, once a plug valve is in field service, such replacement is often difficult. Moreover, the special tooling and manufacturing techniques required in the assembly of a sleeve type liner in a valve body having body lips make in-field replacement of the sleeve type liners very difficult.
In view of the above, it is clear that prior art plug valves have numerous deficiencies.